It is well known that bark and small limbs can be removed from logs in a debarking device by passing the log between a set of rapidly rotating, upper and lower debarking drums in the debarking device, each of which have a plurality of debarking chains, also called flail chains. The debarking chains repetitively strike the surface of the log with significant force at a high rate of speed, generally from about 250 to as much as 500 revolutions per minute (rpm), or more, effectively tearing away any small limbs and most of the bark, preferably virtually all of the bark, from logs passing through a debarking device in preparation for a chipping operation in which the log is chipped into small wood chips appropriate for further processing in such applications as wood pulp for paper manufacturing, manufacturing of composite products such as chip board, oriented strand board and engineered lumber or further processing for use as biofuels.
During debarking operations, logs are conveyed lengthwise through a debarking device along a predetermined feed plane (see FIG. 1). The upper drum is located above the feed plane and the lower drum is located below the feed plane, each at a distance from the feed plane to allow the debarking chains, or flail chains, together to clear bark and limbs from a majority if not the entire surface of the log. It will be appreciated that each set of debarking chains, associated with respective debarking drums, will preferably reach at least the midpoint of the sides of the log. Some debarking devices have additional debarking drums some of which are fixed drums and some of which may be “floating” drums.
It will be appreciated that in prior designs, debarking chains have generally consisted of a series of oblong chain links that have a limited wear life based primarily upon to the wear that occurs proximate the U-shaped end portions of the respective oblong chain links as those links repetitively whipped about a drum rotating at speed that can exceed 500, 600 or even 750 revolutions per minute (rpm) and repetitively strike the outer surface of the logs. During this process, the links will strike the logs and strike each other as the links recoil following such impacts. The wear life that is projected for each chain dictates frequency of chain replacement on each of the debarking drums. Such replacement requires operators to stop the debarking apparatus for the time required to remove the chains and flip them end for end or replace them with new chains. This creates downtime in which revenue from the production of wood chips that can be sold in open markets for such chips, cannot be generated. Operators typically try to keep a debarking apparatus running as close as possible to 24 hours a day to maximize the return on the investment that is made in the machinery and look upon any potential downtime as a limitation on their ability to maximize this return on investment. For that reason, it will be appreciated that any extension of the projected wear life of a set of debarking chains will maximize this return on investment for debarking operations because any extended use of the debarking chains over the projected wear life generally associate with standard debarking chain will not only reduce the number of chains used per unit of logs debarked or wood chips generated, but also reduce the downtime required to replace a set of debarking chains when compared to effective run time.
In normal use, a set of debarking chains are generally attached to a debarking drum, used for an amount of time based on a projected wear life based on historical use under similar conditions or until links break or are worn to a predetermined amount of wear near the point of breakage based on periodic inspection of the debarking chain. When a determination is made that the chains should be flipped end for end or changed out, then the engines are turned off or idled, the chains are all disconnected, and either flipped end for end, before being reattached and used again, or removed and changed out for another set of chains.
Furthermore, even when an operator's projections are appropriate for such chain, chain links from one source or another may have defects that lead to breakage that is premature and therefore unpredictable, which can reduce an operators willingness to place reliance on chain from such a source. It will be appreciated that while an operators ability to predict how long a debarking apparatus can continue to operate without stopping the machines to flip or replace the chains is important, the operator may also try to maximize the amount of wood chips that a produced with any one set of debarking chains, which gives the operator an incentive to maximize the length of time that a set of debarking chains are used. As operators know, however, the downside to stretching the amount of time that a set of debarking chains is used beyond the projected wear life creates an additional risk that a chain will break and go into the chipper that generally follows the debarking drums. If the chains break and go into the chipper they may cause significant damage to the knives in the chipper and reduce the efficiency of the chipper so that repairs to the chipper are required, creating further expense and resulting in further downtime when revenue from wood chip production is not generated.
It is also important to appreciate that the quality of the debarking process is especially important in debarking/chipping operations, because effective debarking will minimize the residual amount of bark mixed into wood chips from subsequent chipping operations. Because wood chips containing even a little bark are much less desirable for downstream use of the chips such as, for instance, pulping operations, operators are docked by buyers of the wood chips for wood chips that contain any amount of bark.
As noted above, a greater fear in debarking/chipping operations is that the debarking chains will break and damage the chipper. This is one of the key reasons that operators carefully avoid extending the use of a set of debarking chains beyond the projected wear life for the chains. If any of the chains break and any of the metal links pass into the chipper, which is generally operating at a very high rate of speed, any consequent damage to the teeth or cutting blades in the chipper can result in added operating costs due to machine downtime and needed replacement parts for needed repairs. Additionally, there exists the risk that breaking links may cause link debris to enter adjacent machinery, causing further damage and downtime.
For these and other reasons, it will be appreciated that there is a practical need to increase projected debarking chain wear life, thereby reducing chain replacement cost per unit of processed wood products.